The following is what I would recommend your basic network layout to be:
- You would need at least one Ethernet cable to go between the Internet modem and the AirPort Extreme.
- You can run one Ethernet cable between the Extreme to an Ethernet switch in the living room.
- The two Macs will be connected to this switch by Ethernet.
- You can run a second Etherent cable between the Extreme and your son's bedroom.
- You can run a third Ethernet cable between the Extreme to an Ethernet switch in your bedroom.
- You can connect the Apple TV directly to this switch.
In addition with this setup, if you want Wi-Fi in areas that are not covered by the Extreme, you can add an Express to any of those switches to do so.
A very good plan to use an Ethernet "backbone" for the network, which will result in much faster and more reliable performance.
The 5th Gen AirPort Extreme was discontinued in June 2013.
The 802.11ac version of the AirPort Extreme offers quite a bit more performance, even if you do not have 802.11ac devices at the present time.
Apple added two additional antennas in this version and moved all of the antennas up to the top of the "tower" for more efficient broadcast operation.
The 2nd Gen Express would really limit performance since it uses 100 Mbps ports. Your Ethernet cabling and the AirPort Extreme model that you choose offer Gigabit or 1,000 Mbps ports. The numbers speak for themselves.
If the budget allows, two AirPort Extremes would be the way to go.
Apple tends to introduce new models in June of most years, so if you are not in a rush, you might want to wait and see what might be coming soon.